GEOSECS Atlantic ³²Si profiles

Abstract

Measurements of five cosmogenic ³²Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO₂ extracted range from 2 to 54 g; the amounts of water from which SiO₂ was extracted range between 540 kg and 270, 000 kg. In additon, SiO₂ recovered from four surface particulate composites (64°N to 61°S) were also analyzed for ³²Si. ³²Si measurements were made by milking and counting the daughter activity, ³²P. The net ³²P activities range from 0.7 to 6.8 cph; typical errors in measurements of the ³²P activities are 20-30%. The ³²Si concentrations vary from 0.6 dpm/106 kg of water in the North Atlantic surface waters to 235 dpm/10⁶ kg at 400 m depth in the circumpolar waters. The vertical profiles of ³²Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and ³²Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of ³²Si (³²Si/SiO₂ ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2-4 times, than the bottom water values. These data are consistent with generation of the highest specific activity ³²Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic ³²Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high ³²Si specific activities. The ³²Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1-1.4)×10⁻² dpm ³²Si/cm², which is consistent with the expected fallout of cosmogenic ³²Si. However, the ³²Si column inventories south of 40°S are higher by a factor of ~5-7, whereas the corresponding Si inventories increase by only a factor of 3. This excess ³²Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of ³²Si dissolved from sinking particulates.